Various means for storing a significant quantity of ink in an ink bag or other container in an ink-jet printer and supplying it in smaller quantities to an ink bladder are known in the prior art. For instance, an ink delivery system has been developed which is provided with a reservoir for supplying a refillable bladder. The bladder is then used to feed the printhead, and when the bladder is depleted, it is refilled from the reservoir, or ink bag. The system utilizes a three-way valve which permits selective fluid communication between the ink bag and the bladder (refill mode) and between the bladder and the ink-jet printhead (print mode). A third position (shipping mode) prevents fluid communication between any of the compartments. U.S. Pat. No. 4,714,937 describes and claims this system.
In the embodiment illustrated in the afore-mentioned patent, the ink bag and the ink bladder are shown mounted side by side on a support platform; the three-way valve and fluid communicating channels are located under the top surface of the platform. The ink flows out of the bottom of the ink bag and into a short vertical channel; then it flows horizontally to the three-way valve; next, with the valve in the refill mode, it flows through the valve into a vertical channel and up into the ink bladder. When the valve is turned to the print mode, the ink flows out of the bottom of the bladder through the vertical channel to the three-way valve, then through the valve into another channel to the printhead.
In another arrangement, not illustrated in the patent but illustrated herein, the ink bag is mounted on the platform and the ink bladder is suspended from the platform and is at a level below the ink bag. With the valve in the refill mode, ink flows out of the bottom of the ink bag into a vertical channel, then into the three-way valve where it is diverted into a horizontal channel to the ink bladder. When the three-way valve is turned to the print mode, ink returns through the horizontal channel to the three-way valve where it is now diverted to another channel to the printhead.
A problem associated with both of these systems occurs when the ink supply is depleted. This allows the air in the ink bag to enter the bladder, from which the air needs to be removed. This task increases the complexity and cost of the product.
In a copending application Ser. No. 378,354, filed on July 11, 1989 and assigned to the same assignee as this application, a sensor system is described and claimed. In a system like the one set out in FIG. 1 of the copending application, a dimpler sets a predetermined back pressure in the system. It also acts as a plunger on the bladder to purge any air trapped in the bladder back into an ink bag. An electrical or mechanical sensor mounted on the dimpler will sense the presence of the bladder in the undimpled mode. If it does not find the bladder, the machine knows that the bladder is out of ink. Then, the three-way valve is rotated to the refill mode and the bladder is refilled. As the bag empties, it collapses. When the bag is emptied, the dimpled bladder can not refill because of a hydraulic lock.
However, it is common for air to be present in the ink bag and to accumulate in the bladder, which is an elastomer. When this happens, the bladder will refill with air rather than ink. The out-of-ink sensor on the dimpler will indicate a bladder full of ink, even though air may be present. This results in a printing failure.